In recent years, with the spread of portable electronic devices such as cell phones, notebook personal computers and the like, there is a large need for development of compact and lightweight secondary batteries having a high energy density. Lithium ion secondary batteries exist as such secondary batteries. Lithium transition metal composite oxide and the like are used as cathode materials of the lithium ion secondary battery which is a non-aqueous electrolyte secondary battery, and lithium metal, lithium alloy, metal oxide, carbon material and the like are used as anode material. These materials are the materials where lithium can be desorbed and adsorbed.
Currently, much research and development is being performed for such lithium-ion secondary batteries. Among these, lithium ion batteries using lithium transitional metal containing composite oxide, especially lithium ion secondary battery using lithium cobalt composite oxide (LiCoO2), for which the composition is comparatively simple, as the cathode material can obtain a 4V class high voltage, so it is being practically used as a battery having a high energy density. Currently, development of lithium nickel composite oxide (LiNiO2) that uses nickel that is less expensive than cobalt, lithium nickel cobalt manganese composite oxide (LiNi0.33Co0.33Mn0.33O2), lithium nickel manganese containing composite oxide including at least nickel and manganese and the like, have been advanced.
Among these, lithium nickel manganese containing composite oxide is relatively less expensive and well balanced in thermal stability and durability and so on, so it catches an attention as a cathode active material. However, the discharging capacity is inferior to that of lithium nickel composite oxide, so lithium nickel manganese containing composite oxide is needed to improve its discharging capacity and packing efficiency in order to improve the energy density. Further, it is also required to have excellent cycling characteristics.
In order to obtain a high energy density and high cycling characteristics, it is effective to make a cathode active material to have a small particle size and a narrow particle size distribution. When a cathode active material having a wide particle size distribution is used, due to the ununiformity of a voltage applied to a particle within an electrode, fine particles selectively deteriorate by repeated charging and discharging, and the discharging capacity lowers. Furthermore, due to the faster deterioration of the discharging capacity, the cycling characteristics become lower.
Generally, a cathode active material of lithium nickel manganese containing composite oxide is manufactured as nickel manganese containing composite hydroxide being its precursor, in order to make a cathode active material being composed of particles having a small particle size and a narrow particle size distribution, it is required to make the nickel manganese containing composite hydroxide that is to be a precursor being composed of particles having a small particle size and a narrow particle size distribution as well.
For example, JP2004-210560 discloses a manganese nickel composite hydroxide that has a substantial manganese-nickel content ratio of 1:1, an average particle size within the range of 5 μm to 15 μm, a tap density within the range of 0.6 g/ml to 1.4 g/ml, a bulk density within the range of 0.4 g/ml to 1.0 g/ml, a specific surface area within the range of 20 g/m2 to 55 g/m2, a sulfate radical content within the range of 0.25% by mass to 0.45% by mass, and the ratio (l0/l1) of the maximum intensity at the peak of X-ray diffraction (l0) within the range of 15≤2θ≤25 and the maximum intensity (l1) within the range of 30≤2θ≤40 of 1 to 6. Further, the surface and internal structure of the secondary particle is netlike due to the pleated wall of the primary particles, and it is said that the space surrounded by the pleated wall is relatively large.
JP2004-210560 further discloses a coprecipitation of particles being produced by reacting a mixed aqueous solution of manganese salt and nickel salt in which the atomic ratio of manganese and nickel is substantially 1:1 with an alkaline solution in a complexing-agent-added aqueous solution having a pH of 9 to 13 under a suitable stirring condition while the oxidation of manganese ion is controlled within a certain range.
However, although JP2004-210560 considers the particle structure of a lithium manganese nickel composite hydroxide, as it is apparent from the disclosed pictures of electron microscope, the obtained particles include large and fine particles and the homogenization of its particle size is not considered.
On the other hand, regarding the particle size distribution of lithium transition metal composite oxide, for example, JP2008-147068 discloses a lithium transition metal composite oxide in which an average particle size D50 meaning the particle size at a cumulative frequency of 50% is within the range of 3 μm to 15 μm, minimum particle size is more than 0.5 μm, maximum particle size is less than 50 μm, and, in a relation of D10 and D90 meaning the particle size at the cumulative frequency of 10% and 90%, D10/D50 is within the range of 0.60 to 0.90, and D10/D90 is within the range of 0.30 to 0.70. This lithium transition metal composite oxide has a high packing efficiency, excellent charge and discharge characteristics and output characteristics, and it is difficult to deteriorate even under a condition with large charge and discharge loads, so when this lithium transition metal composite oxide is used as a cathode material, a non-aqueous electrolyte secondary battery having excellent output characteristics and with little deterioration of cycling characteristics may be obtained.
However, the lithium transition metal composite oxides being disclosed in JP2008-147068 includes fine particles and large particles as the minimum particle size is more than 0.5 μm and the maximum particle size is less than 50 μm with respect to the average particle size of 3 μm to 15 μm. Further, regarding the particle size distribution being defined by D10/D50 and D10/D90, the particle size distribution of the lithium transition metal composite oxide is not narrow. Therefore, even if a cathode active material having an insufficient uniformity of particle size like this is used, it is difficult to sufficiently improve the electrical characteristics of a non-aqueous electrolyte secondary battery.
Further, in order to improve the particle size distribution, many suggestions have been proposed regarding the manufacturing method of a transition metal composite hydroxide as a precursor of a cathode active material. For example, JP2003-086182 proposes a method for obtaining a precursor which is a transition metal composite hydroxide or a transition metal composite oxide, by putting an aqueous solution that includes more than two kinds of the transition metal salt or by putting more than two kinds of aqueous solution of different transition metal salt simultaneously with alkaline solution into a reaction tank, and by coprecipitating them while a reducing agent is made to coexist or while ventilating inert gas.
However, as this technology collect produced crystals while classifying, it would be needed to strictly manage the manufacturing condition in order to obtain products having a uniform particle size. Therefore, the production on an industrial scale which employs this technology is difficult. Further, with this technology, although it is possible to obtain crystal particles having a large particle size, it is difficult to obtain crystal particles having a small particle size.
Further, WO2012/169274 discloses a cathode active material that has: a layered hexagonal lithium nickel composite oxide; an average particle size within the range of 8 μm to 16 μm; and an index of [(D90-D10)/average particle size], which indicates the spread of the particle size distribution, less than 0.60. As such a manufacturing method, it has been proposed to perform nucleation while controlling the pH to be 12.0 to 14.0 at a solution temperature of 25° C. as a standard, then the produced particles are grown by controlling the aqueous solution for particle growth which includes the formed nuclei to have a pH of 10.5 to 12.0 at a solution temperature of 25° C. as a standard as well as by controlling the pH to be lower than that of at the nucleation process, while controlling the stirring power requirement per unit volume at least at the nucleation process to be 0.5 kW/m3 to 4.0 kW/m3. This technology somewhat improves the packing efficiency and output characteristics by homogenizing the particle size distribution, but further improvements should be made regarding the packing efficiency.
On the other hand, JP2003-151546 proposes a cathode active material being composed of hexagonal post shaped particles in order to improve the packing efficiency by focusing on the particle properties of a cathode active material. Although this cathode active material shows an excellent packing efficiency, it is not suitable for industrial manufacture as it requires performing sintering and powdering more than twice. Further, square and plate shaped particles are needed to be grown in a way that its crystal surface is grown specifically in order to obtain desired electrical characteristics, and there is a disadvantage as the quality is not stable. Furthermore, when applying cathode material to produce the electrode, it is very difficult to apply it to make its crystalline orientation suitable. Therefore, in order to achieve a high densification due to a high crystalline orientation, a special application process is required so that the manufacturing cost will be expensive, which is a problem.
Further, JP2003-051311 proposes to separate particles having large particle size and particles having small particle size and adjust the mixing ratio of these particles to suitably mix particles having different particle sizes to have an overall high packing efficiency to obtain a cathode active material that has both excellent rate characteristics and discharging capacity. However, as this technology requires producing two kinds of particles that have different particle sizes, there is a problem that the manufacturing cost becomes high and small particles having small particle size selectively deteriorate.
[Patent Literature][Patent Literature 1]JP2004-210560[Patent Literature 2]JP2008-147068[Patent Literature 3]JP2003-086182[Patent Literature 4]WO2012/169274[Patent Literature 5]JP2003-151546[Patent Literature 6]JP2003-051311